Screen changers are known that may be brought in and out of an operating position by axially displacing a pin or plate in a housing. EP 1,044,094 [U.S. Pat. No. 6,500,336], for example, describes a screen changer in which two filter disks are provided in a slider used as a screen holder. The screen may be exchanged as soon as it exits the housing. It is disadvantageous that a high expenditure of force is necessary to axially displace the screen holder pin, i.e. to laterally move the slider. In such screen changers a screen change occurs infrequently, i.e. at large time intervals of several days to a week, as the result of which the screen holder often becomes encrusted in the housing, so that it moves sluggishly and only with great difficulty.
A multiport valve devoid of dead space is known from EP 1,167,844 [U.S. Pat. No. 6,550,497] for distributing and/or dividing at least one charge of fed plastic melt during production and processing of same, in which the melt flows through all inner chambers of the valve during each operating state. However, such a design is very complicated.
DE 102 34 228 [U.S. Pat. No. 7,011,112] definitively discloses a start-up valve for an extruder having a double passage centrally located in the valve pin. In normal operation, however, the overlap between the valve pin and the housing in the region of the start-up passage is so small that leaks always occur. A secure seal is not ensured in this case. As a result of the central location of the bore in the pin, much material remains in the pin itself as well as on the housing, and this material must be heated to ensure that the start-up valve operates reliably. The start-up valve requires costly materials for its manufacture, and has a high energy demand for operation.
A screen changer for extruders is known from DE 2,844,246 which likewise operates by means of rotary pins having central bores, whose diameters correspond to the feeding discharge passages and which are situated in the screen packs above screen support plates. Rotation of the rotary pin by 90° allows the screen packs to be removed through the closable opening. Here as well, the overlap between the rotary pin and the housing in the region of the removal opening is extremely small, so that leaks always occur during operation of the filter which, however, may be collected by a trap. Here as well, a large amount of material must be heated, which entails significant energy costs.